fix(copilot): guard empty models[].url in custom endpoint and show clear config error

[Birarpanjot Singh Kanwer (arpankanwer)]

Fixes #333701

Problem

Custom Endpoint wizard saves endpoint as name leaving models[].url="".
createOpenAIEndPoint then calls url.includes() on empty/undefined → TypeError: Cannot read properties of undefined (reading 'includes') at extension.js:1598:8522

// Bad - wizard writes
{ "name": "http://localhost:20128/v1", "models": [{ "url": "" }] }

Fix

• hasExplicitApiPath / inferApiTypeFromUrl handle undefined → false / 'chat-completions'
• resolveCustomEndpointUrl validates http(s) and throws clear error for missing url
• createOpenAIEndPoint checks rawUrl = modelConfiguration.url ?? model.url ?? configuration.url and throws actionable error pointing to chatLanguageModels.json and Custom Endpoint wizard saves the endpoint as the group name and leaves models[].url empty #333701

Test

Repro with bad JSON:

• Before: TypeError at aFi(...extension.js:1598:8522)
• After: Custom endpoint model 'or-free' is missing required 'models[].url'. Please configure a valid http(s) URL in chatLanguageModels.json...

Validated:

• node /tmp/repro_333701_without_fix.js → TypeError
• node /tmp/repro_333701_with_fix.js → clear error + valid URLs still resolve correctly

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Birarpanjot Singh Kanwer (arpankanwer)]

@microsoft-github-policy-service agree

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.